This invention relates to flow control systems for use in liquid chromatographs and provides for constant flow of a multi-component carrier liquid under either isocratic or gradient elution modes of operation of the system.
In liquid chromatographic systems used for isocratic and gradient elution, a carrier liquid is used as a mobile phase into which is injected a sample which is transported by the carrier to a stationary phase. Generally, the stationary phase is mounted or packed within a column (LC column) one end of which is provided with a sample injector and connected to the output of a pump for delivering the carrier under pressure thereto. In current high pressure systems the carrier is typically delivered at pressures in the range of about 1,000 to 7,000 psi. The carrier phase in isocratic work contains a fixed proportion of components (for example, 50% water and 50% methanol) while the carrier phase in gradient elution is programmed to vary between the desired values of proportions (for example, from a 90% water/10% methanol to 10% water and 90% methanol) over a predetermined interval of time. In high pressure liquid chromatography (HPLC) the user requires the system to supply a constant flow rate through the column and known proportions or gradient elutions according to settings made in the system and this has been difficult to achieve, particularly in one pump, multiple chamber systems.
It is a primary object of the present invention to provide a liquid chromatographic system which will provide a uniform flow rate through the LC column in both isocratic or gradient operation in accordance with the preset flow rate demand.
In general, the present invention proposes a control system which is particularly adapted to use in multiple chamber single pump systems in which a cam driven by a suitable speed control device such as a stepping motor is connected to a multiple chamber positive displacement piston pump arranged with its chambers and associated pistons in opposition to each other on each side of the cam through an output and fill strokes during each cycle. In a typical such pump the pulsating output strokes of each chamber of the pump are added together and the design of the cam is such that the output strokes overlap and when added through parallel connected output check valves and working into zero pressure, deliver a constant flow for a given drive speed. The inlets to the pump chambers are also connected in parallel through suitable check valves to the output of a composition control module for supplying the carrier liquid in whatever isocratic or gradient proportions selected. The output pressure and flow at the pump when working into a no-load condition is relatively constant. However, in actual systems the pump works against the aforementioned high pressures which causes a non-linear variation in the output of the pump both in the pressure delivered from the pump and in the flow rate which can be produced through a column. This variation is a function of the compressibility and compliance of all of the components involved including the check valve and pump assembly and the compressibility of the solvent carrier components themselves which is a function of several variables including nature of any dissolved gases, and the particular mixture ratios. In themselves, these can be relatively unknown for any arbitrary sets of solvents making up the carrier phase. In addition, the viscosity of the carrier components varies as a function of time in gradient elution work. Also, as the LC column becomes used and can become partially clogged and change its resistance to flow during sample injection and analysis. Many approaches have been taken to controlling the flow through liquid chromatographic systems, examples of which include maintaining a pump motor speed at a constant level as illustrated in the Allington patent U.S. Pat. No. 3,398,689 issued Aug. 27, 1968 and entitled APPARATUS FOR PROVIDING A CONSTANT RATE TO COMPONENT FLOWSTREAM; and by use of a flow feedback loop as illustrated in the Magnussen U.S. Pat. No. 3,917,531 entitled FLOW RATE FEEDBACK CONTROL CHROMATOGRAPH issued Nov. 4, 1975. In general the techniques in the disclosed patents are not adequate to compensate for the pulsation in flow where two or more carrier components are combined and are supplied through a single pump to an LC column. Such pulsations can be caused by mechanical compliance and compressibility or the system, for variations in carrier proportion, and LC column resistance variations; particularly in gradient work wherein the relative viscosity and compressibility of the solvent carrier composition changes as a function of time. There is, therefore, a need for a new and improved flow control system for use in liquid chromatographic systems which will overcome the above limitations and disadvantages.